Deciphering Multiple Critical Parameters of Polymeric Self-Assembly by Fluorescence Spectroscopy of a Single Molecular Rotor BODIPY-C12

Understanding associative polymer self-assembly with shrinking gate fluorescence correlation spectroscopy

The self-assembly of polymers is integral to their role in liquid formulations. In this study, we combine a dye whose lifetime is sensitive to the nanoviscosity of its local environment with shrinking gate fluorescence correlation spectroscopy (sgFCS) to study the self-assembly of a model telechelic polymer, hydrophobically modified ethoxylated urethane (HEUR). Fluorescence lifetime measurements show a monotonic increase in average lifetime with increasing HEUR concentration driven by a small fraction of dye (<1%) with long lifetimes strongly bound to HEUR. Despite this small fraction, sgFCS isolates the diffusional dynamics of the bound fraction with no a priori assumptions as to the distribution of lifetimes. Sensitivity is greatly enhanced compared to standard FCS, revealing micellar aggregates forming between 0.2 and 1 wt% followed by formation of a percolated network. This sgFCS approach, which we apply for the first time to polymers in this work, is readily extendable to any dye that changes lifetime on binding.

Manipulation of Bowl-Shaped Nanoparticles Self-Assembled from a Bipyridine Pendant Containing Homopolymer

The morphological control and transformation of soft nanomaterials are critical for their physical and chemical properties, which can be achieved by dynamically regulating the hydrophilicity of amphiphilic polymers during self-assembly. Herein, an amphiphilic homopolymer poly(N-(2,2'-bipyridine)-4-acrylamide) (PBPyAA) with bipyridine pendants is synthesized, and the effect of various parameters including initial concentration, temperature, pH, and metal ion coordination on the self-assembly behavior and morphology of the assemblies is investigated. Upon changing the initial concentration of PBPyAA, bowl-shaped nanoparticles (BNPs) with precisely controlled diameter, opening size, and thickness are obtained. With the decrease of pH of the solution, the negatively charged surface of BNPs transforms to a positively charged state. Furthermore, the addition of divalent metal ions (Co2+, Mn2+, and Zn2+) induces the transformation of BNPs to vesicles and giant vesicles. The effect of the above factors on the morphology of the assemblies is essential to change the hydrophilicity of PBPyAA dynamically, leading to variation of the local viscosity during self-assembly. Overall, manipulation of the structural parameters of BNPs and transformation of BNPs to vesicles are achieved, providing fresh insights for the precise control of the morphologies of soft nanomaterials.

Fluorescence-readout as a powerful macromolecular characterisation tool

The last few decades have witnessed significant progress in synthetic macromolecular chemistry, which can provide access to diverse macromolecules with varying structural complexities, topology and functionalities, bringing us closer to the aim of controlling soft matter material properties with molecular precision. To reach this goal, the development of advanced analytical techniques, allowing for micro-, molecular level and real-time investigation, is essential. Due to their appealing features, including high sensitivity, large contrast, fast and real-time response, as well as non-invasive characteristics, fluorescence-based techniques have emerged as a powerful tool for macromolecular characterisation to provide detailed information and give new and deep insights beyond those offered by commonly applied analytical methods. Herein, we critically examine how fluorescence phenomena, principles and techniques can be effectively exploited to characterise macromolecules and soft matter materials and to further unravel their constitution, by highlighting representative examples of recent advances across major areas of polymer and materials science, ranging from polymer molecular weight and conversion, architecture, conformation to polymer self-assembly to surfaces, gels and 3D printing. Finally, we discuss the opportunities for fluorescence-readout to further advance the development of macromolecules, leading to the design of polymers and soft matter materials with pre-determined and adaptable properties.

Unraveling an Alternative Mechanism in Polymer Self-Assemblies: An Order-Order Transition with Unusual Molecular Interactions between Hydrophilic and Hydrophobic Polymer Blocks

Polymer self-assembly leading to cooling-induced hydrogel formation is relatively rare for synthetic polymers and typically relies on H-bonding between repeat units. Here, we describe a non-H-bonding mechanism for a cooling-induced reversible order-order (sphere-to-worm) transition and related thermogelation of solutions of polymer self-assemblies. A multitude of complementary analytical tools allowed us to reveal that a significant fraction of the hydrophobic and hydrophilic repeat units of the underlying block copolymer is in close proximity in the gel state. This unusual interaction between hydrophilic and hydrophobic blocks reduces the mobility of the hydrophilic block significantly by condensing the hydrophilic block onto the hydrophobic micelle core, thereby affecting the micelle packing parameter. This triggers the order-order transition from well-defined spherical micelles to long worm-like micelles, which ultimately results in the inverse thermogelation. Molecular dynamics modeling indicates that this unexpected condensation of the hydrophilic corona onto the hydrophobic core is due to particular interactions between amide groups in the hydrophilic repeat units and phenyl rings in the hydrophobic ones. Consequently, changes in the structure of the hydrophilic blocks affecting the strength of the interaction could be used to control macromolecular self-assembly, thus allowing for the tuning of gel characteristics such as strength, persistence, and gelation kinetics. We believe that this mechanism might be a relevant interaction pattern for other polymeric materials as well as their interaction in and with biological environments. For example, controlling the gel characteristics could be considered important for applications in drug delivery or biofabrication.

Imaging Block-Selective Copolymer Solvation

Understanding individual-block solvation in self-assembled block copolymer systems is experimentally difficult, but this solvation underpins the assembly and disassembly observed at the bulk scale. Here, covalently attached viscosity-sensitive molecular rotors for fluorescence lifetime imaging microscopy uncover and quantitatively elucidate previously undisclosed differential block-selective responses toward solvation changes upon addition of DMSO and THF to self-assembled ROMP-based amphiphilic block copolymers. The sensitivity of this method provides unique information on block-selective solvent-triggered assembly and disassembly mechanisms, revealing behaviors invisible to or with superior sensitivity to traditional ¹H NMR spectroscopy. These experiments demonstrate an analytical method and provide a granular mechanistic understanding, both suitable for fine tuning block copolymer assembly and disassembly processes.

Phototoxicity of BODIPY in long-term imaging can be reduced by intramolecular motion

For long-term live-cell fluorescence imaging and biosensing, it is crucial to work with a dye that has high fluorescence quantum yield and photostability without being detrimental to the cells. In this paper, we demonstrate that neutral boron-dipyrromethene (BODIPY)-based molecular rotors have great properties for high-light-dosage demanding live-cell fluorescence imaging applications that require repetitive illuminations. In molecular rotors, an intramolecular rotation (IMR) allows an alternative route for the decay of the singlet excited state (S₁) via the formation of an intramolecular charge transfer state (CT). The occurrence of IMR reduces the probability of the formation of a triplet state (T₁) which could further react with molecular oxygen (³O₂) to form cytotoxic reactive oxygen species, e.g., singlet oxygen (¹O₂). We demonstrate that the oxygen-related nature of the phototoxicity for BODIPY derivatives can be significantly reduced if a neutral molecular rotor is used as a probe. The studied neutral molecular rotor probe shows remarkably lower phototoxicity when compared with both the non-rotating BODIPY derivative and the cationic BODIPY-based molecular rotor in different light dosages and dye concentrations. It is also evident that the charge and localization of the fluorescent probe are as significant as the IMR in terms of the phototoxicity in a long-term live-cell imaging.

Rigidochromism by imide functionalisation of an aminomaleimide fluorophore

Fluorescent dyes that exhibit high solid state quantum yields and sensitivity to the mechanical properties of their local environment are useful for a wide variety of applications, but are limited in chemical diversity. We report a trityl-functionalised maleimide that displays rigidochromic behaviour, becoming highly fluorescent when immobilised in a solid matrix, while displaying negligible fluorescence in solution. Furthermore, the dye's quantum yield is shown to be sensitive to the nature of the surrounding matrix. Computational studies reveal that this behaviour arises from the precise tuning of inter- and intramolecular noncovalent interactions. This work expands the diversity of molecules exhibiting solid state environment sensitivity, and provides important fundamental insights into their design.